2 Answers
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Remove the lens. Newer lenses attach with a bayonet mount and a release may need to be pressed before it can be turned this way. Older lenses used a screw-mount and turning simply unscrew them.

Changing focus. Turning the focus-ring change the distance at which a lens is focused. This is not possible on all lenses and some have to be set to Manual mode first.

Changing aperture. Some modern lens and most old lenses have an aperture ring. If you turn it, it may directly change the aperture or set the aperture to be stopped down to when a photo is taken. If there is an A position, it lets the camera select the aperture.

Zoom: The lens has a zoom ring, then turning it changes its focal-length. Some older zooms use a push-pull mechanism instead of turning.